The Golgi matrix is a collection of proteins involved in the structure and function of the Golgi apparatus. The matrix was first isolated in 1994 as an amorphous collection of 12 proteins that remained associated together in the presence of detergent and 150 mM NaCl. Treatment with a protease enzyme removed the matrix, which confirmed the importance of proteins for the matrix structure. Modern freeze etchelectron microscopy clearly shows a mesh connecting Golgi cisternae and associated vesicles. Further support for the existence of a matrix comes from EM images showing that ribosomes are excluded from regions between and near Golgi cisternae.
Structure and function
The first individual protein component of the matrix was identified in 1995 as Golgin A2. Since then, many other golgin family proteins have been found to be in the Golgi matrix and are associated with the Golgi membranes in a variety of ways. For example, GMAP210 has an ALPS motif in the N-termal 38 amino acids and an ARF1-binding domain called GRAB at the C-terminus. Thus, the GRAB-domain can bind indirectly to Golgi cisternae and its ALPS motif can tether vesicles. Golgins have coiled-coildomains and are thus predicted to have elongated structures up to 200 nm in length. Most are peripheral membrane proteins attached at one end to Golgi membranes. They have flexible regions between the coiled-coil domains, which make them ideal candidates for mediating the dynamic vesicle docking to Golgi cisternae and dynamic structure of the Golgi itself. Golgi reassembly-stacking proteins are an evolutionarily conservedfamily of proteins in the Golgi matrix. GRASP65 and GRASP55 are the 2 human GRASPs. These proteins were named from their requirement for accurate Golgi reassembly during an in vitroassay, but they have also been shown to function in vivo, as shown in the accompanying figure. GRASPs associate with lipid bilayers because they are myristoylated and their myristic acid residue intercalates into the lipid layer. Their trans oligomerization is controlled by phosphorylation and is thought to explain the fragmentation of the Golgi as required during mitosis.
